This invention relates to the manufacture of novel starch-silicate adhesive compositions. In particular this invention concerns the formulation of adhesives for the manufacture of corrugated board by the use of improved mixtures of starches and alkali metal silicates.
The art of using soluble alkali silicates as adhesives for paper and box board has been known for many years. Alkali silicates are inexpensive, fire resistant, vermin resistant, recyclable and provide excellent strength to the products in which they are used. Among other applications these compounds find use as adhesives for paper products and plywood, as well as paper cores and tubes. Alkali silicates do however have a number of disadvantages which have resulted in the discontinuation of their use in certain industrial applications.
A primary disadvantage of silicate based adhesives is that the adhesive bond forms relatively slowly. This is a serious problem in the high speed corrugating machines employed today which require the formation of a strong "green bond" within a very few seconds of the linerboard and the corrugating medium being brought into contact. Another disadvantage of alkali silicates is that they are prone to deposit a tough, intractable glass on any surfaces upon which they are allowed to dry.
Because starch exhibits performance properties which are superior in quick bond formation and does not form a tough deposit, starch has almost completely replaced alkali silicates in the manufacture of corrugated boxboard some years ago. Starch itself, however, is not a perfect adhesive for this application. For one thing, the maximum solids content of such adhesive compositions usually attainable is in the range of 25-30%. Thus some 75% of the adhesive formulation applied to the boxboard consists of water, the removal of which reduces the operational speed of the corrugating equipment, and increases the energy costs associated with water evaporation. The higher water content of the adhesive also contributes to loss of paper strength as well as quality problems during the manufacture of boxes such as warping, blistering or a "washboard" effect.
It has also long been known that the edge crush strength of boxes glued with starch based adhesives is not as high as that which can be attained by use of silicate. These strength properties are now of considerable importance, owing to the fact that the specification of boxes is now based on their edge crush strength (Alternate Freight Regulations Rule 41, as described in Tappi test methods T811) rather than basis weight (as was the case in the past). Use of silicates in the adhesive thus opens the possibility of manufacturing boxes of equivalent edge crush strength either by using paper of a lower basis weight or by using recycled paper of poor quality.
A number of attempts have been made to combine alkali silicates and starch in adhesives for the fabrication of corrugated paperboard boxes and similar applications, in order to realize the benefits described. Thus U.S. Pat. No. 2,669,282 (Kreyling) discloses an adhesive mixture of starch, clay and sodium silicate. U.S. Pat. No. 2,772,996 (Sams) teaches a method of producing such an adhesive by mixing silicate, starch and borax. More recently, Canadian Patent No. 1,056,107 (Falcone, 1979) discloses a class of starch-silicate adhesives in which some of the starch in conventional starch-based adhesive compositions is replaced by alkali silicate, but the total solids content of the adhesive compositions is not appreciably changed.
To date, however, none of the compositions or methods to combine starch and silicate for the manufacture of corrugated boxboard has found commercial application because none has proved to be capable of meeting the demanding requirements of modern high speed corrugating equipment, none of the previous starch and silicate combinations having demonstrated the ability to impart higher edge crush. The principal requisites for such performance features are, firstly, formulation of adhesives so that their viscosity, rheology properties, gelation temperature and speed of "green-bond" formation are all within a very tight range. It is thus essential that the finished adhesive have low thixotropy and a viscosity of between 20-60 Stein-Hall seconds in order to achieve a smooth and rapid transfer of the adhesive from the holding tanks to the corrugator adhesive applicator stations.
A second essential aspect of starch based adhesive formulations is the temperature at which gelation of starch occurs. This usually occurs at a temperature between 60 and 70.degree. C. (140-160.degree. F.), operation of modern corrugating equipment not generally being possible if the gelation temperature is outside this range. Although the aforementioned Falcone patent reveals that addition of sodium silicate to starch leads to a dramatic increase in gel temperature, no teaching has yet been provided of how to formulate satisfactory starch/silicate compositions having gel temperatures low enough to be within an allowable operating range or a viscosity stability over time at elevated temperature, i.e. above 50.degree. F. (120.degree. F.).
There is yet another aspect of starch gelation of pertinence to this invention. During the normal operating procedure the starch based adhesive is maintained in the storage tank at a temperature of between 38-40.degree. C. (100-104.degree. F.). The stability of conventional starch based adhesives is such that this material is prone to premature gelation due to fluctuations in the temperature of the operating environment, and these starch based adhesives can not be stored longer than about 3 days.
We have discovered that by modifying starch-based adhesives by the addition of alkali silicate, in addition to other ingredients such as caustic soda and borax commonly used in starch adhesives, in a precisely controlled and ordered manner, that it is possible to realize all the known advantages of alkali silicates without incurring the problems with viscosity or elevated gelation temperatures which prevented the commercialization of the earlier disclosures.
These formulations are also found to exhibit unusually good high temperature stability. The discovery of high temperature stability has allowed for higher temperature storage which has help offset the higher gel temperature.
Moreover we also discovered, to our surprise, that the methods here described allow the preparation of starch based adhesives with very much higher solids content than previously attainable. As will be demonstrated in the examples below, this discovery increases the solids content of such compositions from the 25-30% range achievable using the conventional technology, to around 45% solids. As mentioned above, reduction of the water content in this manner leads to significant benefits in the operation of corrugating equipment by reducing the strength loss in the liner and mediums caused by water addition from the adhesive, the amount of steam energy required to evaporate the water and the production of boxes with improved dimensional stability.
As the examples given below will show, the invention herein described also lead to improvements both in the strength of the adhesive bond, and in the box itself. The examples also demonstrate that these formulations are entirely compatible with a wide range of operational variables commonly encountered and well known to those skilled in the art.
Formulations according to the present invention are: (i) applicable to raw and modified starches from a wide variety of sources; (ii) compatible with insolubilizing resins such as the cross-linked polymers of melamine-formaldehyde, urea-formaldehyde and ketone-aldehyde commonly used to impart water resistance to starch based adhesives; and (iii) amenable to preparation using conventional techniques for preparing starch-based adhesives, such as the "two-stage" (Example 8, below), "no carrier" (Example 9, below), and "carrier-no carrier" manufacturing processes.
Although most of the examples presented reveal the preparation of the adhesives using liquid ingredients, Example 10 is also presented to demonstrate that similar results can be obtained if only dry ingredients are employed, this being of potential advantage in the commercialization of these materials.
Example 11 below is presented to demonstrate that when these formulations are prepared using recycled water from the corrugating print station (so-called "flexo" water), the concentration of soluble copper in the adhesive is reduced to an unexpectedly low level. The importance of this observation arises from the fact that although the use of flexo water in the preparation of starch based adhesives is an increasingly popular method of recycling waste streams, the conventional starch based compositions employed until now have not been found capable of reducing the solubility of toxic copper ions or other metal ions present in the waste water. The specific reduction of copper observed in the compositions here disclosed is to be attributed to the well known sequestering properties of alkaline silicates.
Although the focus of this disclosure is directed towards manufacture of corrugated boxes, it should also be noted that this is but one of numerous area of potential application of the silicate-starch compositions herein described. Adhesives according to the invention may be used with other cellulosic materials, such as wood and other paper products.